The present invention relates to the general field of aviation.
More particularly, the invention relates to monitoring a servo-control loop of an actuator system for actuating variable-geometry components of a turbojet fitted to an aircraft.
The term “variable-geometry components” is used herein to mean components of equipment other than the rotary members of the engine and that present dimensions, shapes, and/or positions and/or speeds that can be modified as a function of detected events or as a function of engine speeds. Examples of “variable-geometry components” include pitch angles of variable-pitch stator vanes (VSVs) of compressors, variable bleed valves (VBVs) of the compressor, turbine blade tip clearances, and the positions of a fuel metering unit.
In traditional manner, these variable dimensions, shapes, or positions are varied by hydraulic actuator systems that use fuel is their actuating fluid. These actuator systems are controlled by the electronic regulator module or electronic control unit (ECU) of the full authority digital engine control (FADEC) of the aircraft so as to adapt the variable geometries to the flight scenario (or mission) of the aircraft. Control is performed by means of servo control loops.
Thus, it can readily be understood that it is crucial to monitor the actuator systems of variable-geometry components in order to ensure that a turbojet is available and efficient.
Degradations to such actuator systems lead, in particular, to components of the turbojet taking up positions and/or sizes that do not comply with the commands from the ECU under steady conditions, or to said components responding slowly to such commands under transient conditions. Such degradations are early signs of failure. In general, they are initially compensated by the servo-control loops of the actuator systems or else they are without any significant consequence other than a reconfiguration of the actuator systems (e.g. changing the active control channel).
Nevertheless, after a certain amount of time, when such degradations persist and get worse, they are no longer compensated and can make the turbojet inefficient or inoperative. The ECU regulation system can then issue a failure message.
Thus, it can be understood that such deteriorations are detected too late when they are detected only as a result of the actuator system breaking down.
There thus exists a need to have a method of effectively monitoring the servo-control loops of the actuator systems for actuating variable-geometry components of a turbojet, in particular in order to be able to issue a maintenance notice for such actuator systems before the turbojet is made inoperative or inefficient, while taking care to minimize the number of useless or untimely maintenance operations that are performed.
By way of example, document FR 2 939 924 and presently unpublished document FR 10/58681 describe techniques of detecting failures in an aeroengine, the techniques involving estimating a plurality of indicators.
The document “Hydraulic actuation loop degradation diagnosis and prognosis”, by E. Diez-Lledó, J. Aguila-Margin, J-R. Massé, A. Sif, and E. Griful, 1st CEAS European Air and Space conference, 2007, describes the use of diagnosis and prognosis techniques applied to monitoring servo-control loops of systems for actuating variable-pitch stator vanes.